Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder intake port (port fuel injection). Direct injection (DI) offers higher fuel efficiency and higher power output in addition to enabling a charge cooling effect of the injected fuel. However, direct injected engines usually have higher particulate matter emissions (or soot) due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion. Port fuel injection usually (PFI) provides cleaner emissions and high performance under low loads, due to improved mixing. In engine systems configured with each of a port injector and a direct injector coupled to each engine cylinder, a ratio of fuel(s) delivered to a given cylinder via port injection and direct injection can be varied.
One example approach is shown by Bidner et al in U.S. Pat. No. 8,100,107. Therein, a split ratio of fuel injection is adjusted to reduce particulate matter (PM) emissions. Specifically, during selected operating conditions, such as at higher engine speeds and loads, a smaller proportion of port fuel injection and a larger proportion of direct fuel injection is used to take advantage of the higher power output of the more precise direct injection as well as the charge cooling properties of the direct injected fuel. In comparison, at lower engine speeds and loads, a higher proportion of port injection may be used.
However, the inventors herein have identified potential issues with such an approach. The benefits associated with port fuel injection can be a function of the intake valve temperature. Specifically, port fuel injection is used to improve fuel economy benefits due to increased manifold pressure, which arises from fuel evaporated in the cylinder's intake port by absorbing heat from the intake valves. The evaporation of the port injected fuel atomizes the fuel very well, thereby reducing particulate matter emissions. However, there may be conditions at low engine speed-load operating regions where the intake valve temperature is not sufficiently warm. In addition, there may be significant cylinder-to-cylinder variation in intake valve temperature. If a higher proportion of port injection is scheduled for a cylinder where the intake valve is not sufficiently warm, particulate matter emissions may actually be increased. As a result, even with the shift towards more port injection, particulate matter (PM) emissions may not be sufficiently reduced to meet the mandated low PM emission standards. In addition, due to inefficient fuel vaporization, engine performance may be degraded.
In one example, some of the above issues may be addressed by a method for an engine comprising adjusting a ratio of fuel delivered to a cylinder via direct injection relative to port injection based on a temperature of an intake valve of the cylinder. In this way, port injection may be enabled during conditions when port injection benefits can be applied.
As an example, an engine system may be configured with each of a port injector and a direct injector coupled to each engine cylinder. In some embodiments, the port injector may deliver a fuel of a different composition and alcohol content than the fuel delivered via the direct injector. An engine controller may be configured to generate an initial fuel injection profile for all engine cylinders based on operating conditions such as engine speed, combustion event number, exhaust catalyst temperature (e.g., if it is a hot start or a cold start), etc. For example, during an engine cold start, for a first number of combustion events since the engine start, the initial fuel injection profile may include a higher proportion of fuel delivered via direct injection relative to port injection. As such, the initial fuel injection profile may be common to all engine cylinders.
The engine controller may then modify the initial fuel injection for each engine cylinder based on individual cylinder intake valve temperatures. For example, the proportion of fuel delivered via port injection may be increased as the intake valve temperature increases (e.g., exceeds a threshold) to increase the benefits of port injection. In addition, a timing of port injecting fuel may be moved closer towards intake valve opening as the temperature increases. The proportion may also be adjusted based on the fuel being port injected to increase vaporization of the fuel. As a result of the cylinder-specific fuel injection profile customization, there may be cylinders having a lower intake valve temperature operating with a relatively smaller amount of port fuel injection and other cylinders having a higher intake valve temperature operating with a relatively larger amount of port fuel injection. Once a threshold engine speed is attained (e.g., idling speed), all engine cylinders may be transitioned to an idling fuel injection profile.
In this way, the scheduling of a port injector may be adjusted based on the intake valve temperature of a cylinder to improve the port injection benefits. By increasing the fraction of fuel delivered via the port injector as the intake valve temperature increases, the amount of fuel evaporated and homogenized in the intake port is increased. In addition, the time taken to vaporize the fuel is reduced, allowing for adjusting of valve timing. By biasing towards port injection during conditions when port injected fuel can be efficiently vaporized, particulate matter emissions are reduced. In addition, engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.